Polystyrene, such as general purpose polystyrene (GPPS), is made from styrene, a vinyl aromatic monomer that can be produced from aromatic hydrocarbons, for example those derived from petroleum. GPPS is useful in a variety of applications, such as casing for appliances, molded into toys or utensils, or expanded to create foamed styrene. In most cases, GPPS is a hard and brittle plastic, however, the use of comonomers may alter its physical properties, for example, styrene can be copolymerized with polybutadiene to make SBS rubber. The resulting SBS polymer has more rubber-like qualities, such as elastomeric performance and abrasion resistance. Other polymers can also experience altered physical properties when polymerized using comonomers. Ionic comonomers, for example, may alter properties such as melt flow rate, melt strength, polydispersity, and glass transition temperature.
When ionic comonomers are used, the polymer product is an ionomer. An ionomer is a polymer that contains nonionic repeating units and a small portion of ionic repeating units. Generally, the ionic groups make up less than 15% of the polymer. The ionic groups are attached to the polymer backbone at random intervals and can reversibly associate with one another, creating reversible crosslinks. These reversible crosslinks can cause the polystyrene to be less brittle and more resistant to abrasions. Ionic aggregates in the copolymer can also affect such properties as bending modulus, tensile strength, impact resistance, and melt viscosity.
Unsaturated carboxylic acid salts are a group of ionic comonomers that can serve as effective crosslinking agents. Metal methacrylates are an example of carboxylic acid salts. One metal methacrylate that may be useful as an ionomeric crosslinker is zinc dimethacrylate, Zn(MA)2 or ZnDMA. It is a divalent metal and therefore, capable of forming two reversible crosslinks with the ionized acid ends of the methacrylates that are incorporated into the backbones of polystyrene chains.
Zinc methacrylates can be obtained commercially in a powder form or prepared from precursors via a variety of methods. These methods are rather labor-intensive and expensive, and the resulting ZnDMA powder may contain unwanted insoluble impurities. Further, particles of ZnDMA powder tend to be less than 10 microns in diameter, causing inhalation hazards during transport to the reaction vessel due to dust. Further, ZnDMA is polar, and therefore does not easily dissolve into styrene. In the reaction vessel, sticking of the methacrylate particles to the vessel surfaces can occur during mixing due to the static charge build-up on the surface of these polar compounds. Thus, for all of the above reasons, preparation of a ZnDMA powder may not be the best option for incorporating this ionic comonomer into polystyrene production.
It would be desirable to have a technique for the in-situ preparation of ZnDMA as a comonomer in the production of GPPS that would be reproducible, yield a fluid suspension that is readily pumpable and pourable, and give GPPS enhanced physical properties such as high melt strength.